influence, its magnetic moment preferentially aligns with the magnetic field. The magnetic moment of gadolinium is 1,000 times greater than that of a hydrogen nucleus, and its presence in tissues shortens the T1 relaxation

9time, resulting in a marked increase in signal intensity. In many cases, gadolinium will increase the relative contrast of adjacent tissues, and the degree of enhancement can often be used to help characterize specific pathologic lesions (see Table 9.4). However, this enhancing effect of gadolinium on some tissues may actually result in decreased contrast in the orbit because of the intense signal from adjacent retrobulbar fat on routine T1-weighted sequences, so that the lesion may not be distinguished from normal orbital fat. Various fat suppression techniques are available and should be employed for better visualization of gadolinium-enhanced tissues within the orbital fat.

Special techniques can greatly expand the usefulness of MRI in certain circumstances. Time-of-flight MR angiography is based on the phenomenon of flow-related enhancement of spinning protons entering an imaging slice. These “fresh” protons enter unsaturated, thereby giving a higher signal than the surrounding stationary protons. Images can be combined or obtained simultaneously by phase encoding in the slice direction to produce a 3D image of the vessels analogous to a conventional angiogram.

Summary for the Clinician

9.4Imaging of Common Orbital Lesions

It is useful to review the typical CT and MRI appearance of some common orbital lesions as a reference for clinical evaluation. It should be kept in mind that many lesions will have similar imaging characteristics, so that definitive diagnosis is usually not possible. Nevertheless, a carefully selected CT or MRI sequence, or a combination of both modalities, will frequently narrow the differential diagnosis to a few more likely possibilities.

9.4.1Adenoid Cystic Carcinoma

Adenoid cystic carcinoma is the most common primary malignancy of the lacrimal gland, representing about 30% of all epithelial lacrimal gland tumors. It is seen most commonly in the fourth decade of life. The CT usually shows a round-to-oval, heterogeneous mass that is defined to poorly demarcated in the superotemporal orbit. It may extend along the lateral orbital wall, and foci of calcification may be seen. Bone destruction is a frequent finding. On MRI, the T1 image gives a heterogeneous hyperintense signal to muscle that becomes hyperintense to fat on the T2 sequence. Moderate enhancement is seen with gadolinium (Fig. 9.11a, b).

9.4.2Cavernous Hemangioma

The cavernous hemangioma is a benign, noninfiltrative,

cross-sectional slice of the body.

■It relies on resonance signals generated by protons exposed to an external magnetic field.

■Within this magnetic field, when exposed to a RF pulse these spinning protons change their orientation and clustering as they rotate around the mean magnetic direction.

■When the RF pulse is removed, the protons return to baseline by relaxation processes while giving up energy to the environment as T1 and T2 resonance signals.

■These signals can be measured and manipulated to produce an image that can be weighted toward the T1 or the T2 resonance signals.

■Various tissues show different T1 and T2 relaxation times, and these can be used to maximize the signal strengths and therefore the contrast between these tissues.

aged adults from 20 to 60 years of age. On CT scan, these lesions demonstrate a well-defined, rounded, homogeneous tissue density mass. Bone remodeling may be seen with long-standing lesions. Enhancement is mild to moderate owing to generally low vascular flow. The MRI shows a homogeneous isointense signal on T1-weighted images and a high signal on T2WI (Fig. 9.12a, b).

9.4.3Dermoid Cyst

The dermoid is the most common orbital cystic lesion. It represents a developmental choristoma arising from trapped pouches of ectoderm into bony sutures or from failure of surface ectoderm to separate from the neural tube. They slowly enlarge as they fill with sebum and keratin. The CT appearance is a rounded, well-defined cystic lesion usually in the anterior superotemporal orbit, eyelid, or brow. The center typically shows a low fat density,

and a fluid–fat level may sometimes be seen. On T1 MRI images, the cyst shows low signal intensity due to water content but may be hyperintense when there is a high fat content. On T2 images, a fluid–fat interface will show an upper lipid layer with low intensity and a lower fluid layer that is hyperintense (Fig. 9.13a, b).

9.4.4Fibrous Dysplasia

Fibrous dysplasia is a nonhereditary benign developmental fibro-osseous anomaly that represents a

hamartomatous malformation. Bone is replaced with fibrous tissue containing abnormally arranged dysplastic bony trabeculae. Progressive constriction of orbital foramina and canals may cause cranial nerve palsies and visual loss from optic nerve compression. The CT image is best evaluated with bone window settings and shows bone thickening and sclerosis with a typical ground-glass appearance and narrowing of orbital foramina. On MRI, the bone images as homogeneous and hypointense, with less-calcified areas showing foci of more hyperintense signal (Fig. 9.14).

Fig. 9.14 Bone window CT showing the characteristic imaging findings of fibrous dysplasia

9.4.5Lymphangioma

Lymphangiomas are lesions of abortive vascular elements that arborize among normal structures. They represent hamartomas of venous–lymphatic channels. Although these lesions are hemodynamically isolated from largeflow vessels of the arteriovenous system, they are prone to intrinsic hemorrhage from small vessels. The CT scan shows irregular heterogeneous and poorly defined infiltrates among normal orbital structures. Low-density cystic areas may be present, and higher-density phleboliths may be seen. The MRI image demonstrates a mildly

hyperintense heterogeneous signal on T1-WI (T1 weighted image) and on the T2-WI (T2 weighted image) blood cysts give a high signal intensity. Serpentine areas of signal void within the mass represent vessels containing flowing blood (Fig. 15a–c).

9.4.6Lymphoma

The vast majority of orbital lymphomas are of the nonHodgkin variety, mostly low-grade proliferations of small monoclonal B lymphocytes. Lymphomas represent 5–10% of orbital mass lesions and 40–60% of lymphoproliferative disease in the orbit. On CT scan, lymphomas appear as a mass that is diffuse to moderately defined and is homogeneous or less often heterogeneous in texture, isodense to muscle, and typically molded around the globe, the optic nerve, and along the orbital walls. On T1 MRI images, lymphomas are slightly hyperintense with respect to muscle and hypointense to fat. On T2 images, resonance signals are brighter but variable from isointense to moderately hyperintense with respect to both muscle and fat (Fig. 9.16a, b).

9.4.7Myositis

Myositis is an orbital inflammatory process confined to one or more extraocular muscles. It is related to the group of idiopathic inflammatory pseudotumor syndromes of unknown etiology. The CT image shows diffuse enlargement of one or more extraocular muscles with somewhat